Implement file support with models, interface, and comprehensive tests

Co-authored-by: StevanFreeborn <65925598+StevanFreeborn@users.noreply.github.com>

src/AnthropicClient/Models/FileRequest.cs

@@ -0,0 +1,51 @@
+using AnthropicClient.Utils;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a request to create a file.
+/// </summary>
+public class FileRequest
+{
+  /// <summary>
+  /// The file content as a byte array.
+  /// </summary>
+  public byte[] Content { get; }
+
+  /// <summary>
+  /// The filename of the file.
+  /// </summary>
+  public string Filename { get; }
+
+  /// <summary>
+  /// The MIME type of the file.
+  /// </summary>
+  public string ContentType { get; }
+
+  /// <summary>
+  /// The purpose of the file.
+  /// </summary>
+  public string Purpose { get; }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="FileRequest"/> class.
+  /// </summary>
+  /// <param name="content">The file content as a byte array.</param>
+  /// <param name="filename">The filename of the file.</param>
+  /// <param name="contentType">The MIME type of the file.</param>
+  /// <param name="purpose">The purpose of the file (default: "user_upload").</param>
+  /// <exception cref="ArgumentNullException">Thrown when content, filename, or contentType is null.</exception>
+  /// <exception cref="ArgumentException">Thrown when filename or contentType is empty.</exception>
+  public FileRequest(byte[] content, string filename, string contentType, string purpose = "user_upload")
+  {
+    ArgumentValidator.ThrowIfNull(content, nameof(content));
+    ArgumentValidator.ThrowIfNullOrWhitespace(filename, nameof(filename));
+    ArgumentValidator.ThrowIfNullOrWhitespace(contentType, nameof(contentType));
+    ArgumentValidator.ThrowIfNullOrWhitespace(purpose, nameof(purpose));
+
+    Content = content;
+    Filename = filename;
+    ContentType = contentType;
+    Purpose = purpose;
+  }
+}